1. Field of the Invention
The present invention relates to a device and method for charging a secondary battery and, particularly, to a device and method for charging a secondary battery for detecting that the secondary battery is abnormal when charging the secondary battery.
2. Description of the Related Art
A secondary battery is generally a battery which can store power and can supply a direct current to the outside, as necessary, after repeated charges and discharges, and is used as a memory backup for a portable electronics device or computer such as a laptop personal computer, etc. It is known to use, for example, a lithium-ion battery as a secondary battery. A conventional charging device for a secondary battery performs a preliminary charging before a boost charging, and determines that the battery is abnormal when the open-circuit voltage of the battery has not reached a threshold value within a predetermined time from the start of the charging, and then indicates that the battery is abnormal with a LED without performing the boost charging, and determines that the battery is normal when the open-circuit voltage of the battery has reached the threshold value or more within the predetermined time from the start of the charging, and then performs the boost charging. For this purpose, the charging device checks the open-circuit voltage of the battery, repeating the preliminary charging of the battery and the opening of the receiving end of the battery periodically within the predetermined time.
FIG. 4 is a flowchart of the current control of the conventional charging device. When the charging device charges a secondary battery (simply called a xe2x80x9cbatteryxe2x80x9d hereinafter), it controls the charging current so as to perform the preliminary charging at first, and then performs the boost charging after determining that the battery is not abnormal.
At step S101, when the AC power is supplied to the charging device, the preliminary charging is started, and a preliminary charging current of, for example, 100 mA is fed to the battery.
At step S102, it is determined whether a predetermined time of, for example, 10 seconds has passed since the preliminary charging started, and in case of YES the process goes to step S103, but in case of NO the process returns to step S101 to continue the preliminary charging.
At step S103, the terminal of the battery is opened to stop the preliminary charging, and the open state terminal voltage of the battery is measured.
A step S104, it is determined whether the predetermined time of, for example, 1 second has passed since the preliminary charging stopped at step S103, and in case of YES the process goes to step S105, but in case of NO the process returns to step S103 to continue to stop the preliminary charging.
At step S105, it is determined whether the open-circuit voltage measured at step S104 is larger than a predetermined threshold value, and in case of YES the process goes to step S108, but in case of NO the process goes to step S106.
At step S106, it is determined whether the predetermined charging time of, for example, 1 hour has passed since the charging started, and in case of YES the process goes to step S107, but in case of NO the process returns to step S101 to start the preliminary charging again.
At step S107, the LED is lit to indicate that the battery is abnormal, and the charging is finished.
At step S108, the boost charging is performed.
At step S109, it is determined whether the terminal voltage during the boost charging has reached the predetermined voltage, and in case of YES the charging is finished, but in case of NO the process returns to step S108 to continue the boost charging.
However, there is a problem, with the conventional charging device for a secondary battery, in that the charging device checks the open-circuit voltage of the battery, when charging an abnormal secondary battery, repeating the preliminary charging of the battery and the opening of the circuit to the battery periodically within a predetermined time (for example, one hour), but it may happen that the charging device does not detect that the battery is abnormal because the open-circuit voltage does not exceed the threshold value during the predetermined time, and only after it has performed the preliminary charging for a long time over the predetermined time, can it detect the abnormal battery.
In addition, there is a problem, with the conventional device for charging a secondary battery, in that as an abnormal secondary battery may have an open circuit voltage which is over the threshold value, the charging device is not able to surely detect an abnormality of a secondary battery.
It is therefore an object of the present invention to provide a device for charging a secondary battery, which is able to detect an abnormality of a secondary battery exactly and surely in a short time.
In order to achieve the above object, a device for charging a secondary battery according to the present invention comprises: a voltage measuring circuit for measuring the terminal voltage of said secondary battery; a current control circuit for controlling a charging current fed through said secondary battery; and an abnormality determining portion which starts charging said secondary battery, finishes said charging when said terminal voltage has reached a first threshold value which is decided based on the normal-state upper limit of the internal resistance and the charging current of said secondary battery, measures the open-circuit voltage of said secondary battery after finishing said charging, and determines that said secondary battery is abnormal when said open-circuit voltage is less than a second threshold value.
In the above device for charging a secondary battery according to the present invention, said abnormality determining portion determines that said secondary battery is abnormal when said open-circuit voltage is larger than a third threshold value.
In the above device for charging a secondary battery according to the present invention, said abnormality determining portion obtains said internal resistance of said secondary battery based on a first terminal voltage of said secondary battery while being charged and a second terminal voltage of said secondary battery whose terminal is opened after finishing said charging, and determines that said secondary battery is abnormal when said internal resistance is out of an allowable range.
In the above device for charging a secondary battery according to the present invention, said current control circuit performs a boost charging of said secondary battery when said abnormality determining portion has determined that said secondary battery is normal.
In order to achieve the above object, a method for charging a secondary battery, using a voltage measuring circuit for measuring the terminal voltage of said secondary battery, a current control circuit for controlling a charging current fed through said secondary battery, and an abnormality determining portion, comprising steps of: starting the charging of said secondary battery; finishing said charging when said terminal voltage has reached to a first threshold value which is decided based on the normal-state upper limit of the internal resistance and the charging current of said secondary battery; measuring the open-circuit voltage of said secondary battery after finishing said charging; and determining that said secondary battery is abnormal when said open-circuit voltage is less than a second threshold value.
In the above method for charging a secondary battery according to the present invention, the method further comprises a step of determining that said secondary battery is abnormal when said open-circuit voltage is larger than a third threshold value.
In the above method for charging a secondary battery according to the present invention, the method further comprises a step at which said abnormality determining portion obtains said internal resistance of said secondary battery based on a first terminal voltage of said secondary battery being charged and a second terminal voltage of said secondary battery whose terminal is opened after finishing the charging, and determines that said secondary battery is abnormal when said internal resistance is out of an allowable range.
In the above method for charging a secondary battery according to the present invention, the method further comprises a step at which said current control circuit performs a boost charging of said secondary battery when said abnormality determining portion has determined that said secondary battery is normal.
In the above method for charging a secondary battery according to the present invention, the method further comprises a step of indicating that said secondary battery is abnormal when it has been determined that said secondary battery is abnormal.
In the above device or method for charging a secondary battery according to the present invention, setting a first threshold value for finishing the charging of the battery, which is decided based on the normal-state upper limit of the internal resistance and the charging current of the battery, allows the terminal voltage of the battery being charged to be measured without opening the terminal of the battery, and allows the measured terminal voltage to be compared with the first threshold value, and thereby a complicated control, such as repeating the charging and opening the circuit, is not required.
Furthermore, when the terminal voltage of the battery being charged has reached to the first threshold value, and the terminal voltage of the battery whose terminal is opened after finishing the charging is less than a second threshold value for determining that the secondary battery is abnormal, the internal resistance of the battery may be assumed to be at the normal state upper limit or more, and thereby it may be determined that the battery is abnormal.
Furthermore, by obtaining the internal resistance of the battery based on the terminal voltage of the battery being charged and the terminal voltage of the battery whose terminal is opened after finishing the charging, and by determining whether the internal resistance is in the normal state allowable range of the battery, it may be assumed that the internal resistance is larger than the normal state upper limit when the open-circuit voltage is less than the second threshold value for determining that the battery is abnormal, while it may be assumed that the internal resistance is less than the normal state lower limit when the open-circuit voltage is larger than a third threshold value for detecting that the battery is abnormal, and thereby it may be determined that the secondary battery is abnormal, and an abnormal secondary battery may be surely detected.